/Engine/Source/Editor/BlueprintGraph/Public/BlueprintActionFilter.h
Users can extend the filter and add their own rejection tests with this method. We use rejection "IsFiltered" tests rather than inclusive tests because it is more optimal to whittle down the list of actions early.
void AddRejectionTest ( [FRejectionTestDelegate](API\Editor\BlueprintGraph\FBlueprintActionFilter\FRejectionTestDelegate) RejectionTestDelegate )